annotated_commit: add failing test for looking up from annotated tag

tests/merge/annotated_commit.c

+#include "clar_libgit2.h"
+
+
+static git_repository *g_repo;
+
+void test_merge_annotated_commit__initialize(void)
+{
+	g_repo = cl_git_sandbox_init("testrepo");
+}
+
+void test_merge_annotated_commit__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+void test_merge_annotated_commit__lookup_annotated_tag(void)
+{
+	git_annotated_commit *commit;
+	git_reference *ref;
+
+	cl_git_pass(git_reference_lookup(&ref, g_repo, "refs/tags/test"));
+	cl_git_pass(git_annotated_commit_from_ref(&commit, g_repo, ref));
+
+	git_annotated_commit_free(commit);
+	git_reference_free(ref);
+}